1. Field of the Invention
The present invention relates to nitride semiconductor light emitting devices. More particularly, the present invention relates to a multielectrode type nitride semiconductor light emitting device having at least one of a p-electrode and an n-electrode electrically separated into at least two regions.
2. Description of the Background Art
A nitride semiconductor laser which is one example of a nitride semiconductor light emitting device has attracted much research efforts to be employed as a light source for an optical disk system in view of the emission wavelength being approximately 400 nm. On occasions where such a nitride semiconductor laser is employed as the light source of an optical disk system and the like, the problem of the reflected light from the disk surface recombining with the semiconductor laser to cause optical feedback noise is of concern. It is generally known that the coherence can be reduced by setting the carrier density of the semiconductor laser in a transition state to alleviate gain convergence of the oscillation spectrum. It is considered preferable to conduct self pulsation through high frequency superposition modulating injection current, and the interaction between carriers and photons in the semiconductor laser. Particularly, the usage of the latter self pulsation device is advantageous from the standpoint of cost and usability since a high frequency circuit is dispensable.
Japanese Patent Laying-Open No. 9-191160 discloses a low noise semiconductor laser for an optical disk as a nitride semiconductor laser having self pulsation characteristics. A low noise semiconductor laser that is stable by virtue of including a saturable absorption layer with InGaN as the constituent element is provided, as shown in FIG. 14. This low noise semiconductor laser has a structure set forth below. On an n type SiC substrate 400, an n type AlN layer 401, an n type AlGaN clad layer 402, an n type GaN light guide layer 403, an InGaN quantum well active layer 404, a p type GaN light guide layer 405, a p type AlGaN clad layer 406 and a p type GaN contact layer 407 are sequentially formed. An InGaN saturable absorption layer 408 is provided at p type GaN light guide layer 405.
The aforementioned active layer and saturable absorption layer containing In exhibit poor stability in characteristics due to strain by the substrate and variation in the growing conditions. It is to be particularly noted that variation in the characteristics of the saturable absorption layer may degrade the yield of self pulsation. The growth temperature of the saturable absorption layer is reduced to approximately 850° C. at most due to the inclusion of In. This is lower by at least 100° C. than the growth temperature of an adjacent nitride layer containing Ga as the main element or the nitride layer containing Al and Ga as the main elements. The difference in the growth temperature may sometimes become more than 300° C. Thus, there is a possibility that the processing step during deposition will be rendered complicated by the provision of such a saturable absorption layer. Also, the thermal hysteresis during deposition may adversely affect the characteristics of the active layer. It is to be also noted that the light passing through the waveguide will always be susceptible to the saturable absorption layer occupying one entire layer in the multilayer structure.
Japanese Patent Laying-Open No. 1-251775 discloses a multielectrode type semiconductor light emitting device as another semiconductor laser. The active layer of this semiconductor light emitting device is of AlGaAs base, and does not contain N (nitrogen). In this publication, the position relationship between the p-n junction and active layer in a non-bias state and the impurity concentration of the peripheral layer for the semiconductor light emitting device are not clarified. It cannot be said that a general structure is presented. Specifically, possible effects of a saturable absorber in a non-bias state, when the active layer is not spatially overlapped with the depletion layer generated in the p-n junction, include absorption due to current not being injected, and a carrier recombination effect in the active layer. Another possible effect may be promotion of carrier discharge from the active layer by the internal electric field of the depletion layer after some of the carriers are diffused as far as the depletion layer due to the feasible diffusion of carriers of the GaAs-base active layer. It is considered that, for the purpose of obtaining favorable self pulsation, the aforementioned structure is subject to limitation imposed by the material or the structure such as the high differential gain of the active layer, feasible diffusion of carriers, and the like.